Typically, particle detectors employ laser devices producing a beam of coherent light that is directed to impinge upon a sample of particles. Particle detection is achieved either by sensing light scattered by a particle, e.g. U.S. Pat. No. 5,085,500 to Blesener et al., or by detecting extinction of light, e.g. U.S. Pat. No. 5,121,988 to Blesener et al. U.S. Pat. No. 5,317,447 to Baird et al. discloses, in pertinent part, a solid-state laser having a Cr:LiCAIF crystal pumped from one end by an array of tunable semiconductor laser diodes. Tilted birefringent plates are positioned within the solid-state resonator cavity to control the spectral bandwidth and wavelength output. A periodically segmented domain-inverted KTP waveguide is disposed outside of the resonant cavity, in the path of the output beam, to shorten the wavelength of the beam passing therethrough. Despite the recent advances in solid-state laser technology, it has not been applied to particle detectors.
Particle detectors have been used for a variety of purposes to detect the presence and/or size of particles in various fluids, including air and other gases, as well as liquids, such as water, hydraulic oils and lubricants. They have proved particularly useful to control contamination in many industrial environments. For example, particulate contamination can cause hydraulic equipment and the like to fail due to excessive accumulation of particles in the hydraulic fluid. Even though filters are used in such equipment to continuously remove particles, the filters may become clogged and may rupture due to excess pressure build-up across the filter membrane. Also, microelectronic fabrication requires a "clean room" in which particulate contaminants, e.g., dust, are filtered from an atmosphere of a room. The filters used in "clean rooms" are also subject to clogging and compromise, resulting in particulate matter entering a "clean room" atmosphere in great quantities. Failure to provide a "clean room" results in particulate contamination of the devices during fabrication, which reduces yield. Particle detectors are thus used in such environments to detect particles in specified size classes and report the cleanliness level of the fluid according to categories specified by industry standards.
A significant amount of research has been performed using open cavity gas lasers in particle detection systems and is discussed by R. G. Knollenberg and B. Schuster in "Detection and Sizing of Small Particles in an Open Cavity Gas Laser", Applied Optics, Volume 11, Number 7, November 1972, pages 1515-1520. Sub-micron particle sizing devices utilizing light scattering in an open cavity laser device is described by R. G. Knollenberg and R. E. Leur in "Open Cavity Laser `Active` Scattering Particle Spectrometry from 0.05 to 5 Microns", Fine Particles, Aerosol, Generation Measurement, Sampling and Analysis, Editor Benjamin Y. H. Liu, Academic Press, May, 1975, pages 669,696. However, the susceptibility of the open cavity gas lasers to contamination has made them require more frequent servicing to clean the laser optics, leading to the development of the external cavity gas laser.
U.S. Pat. No. 4,594,715 to Knollenberg discloses an external cavity gas laser for use as a particle detector that includes first, second and third spaced mirrors. The first and second mirrors define an active resonant cavity of a gas laser, and the second and third mirrors define a second cavity. The second cavity ranges between being passive and being closely coupled as part of the active cavity, depending on the phase of the light returned from the third mirror to the second mirror. In the limiting case where the second cavity is not resonant, a large field does not build up in the passive cavity, because that cavity is off resonance for the wavelength of the active cavity. In the latter case, the second mirror, ignoring scattering and absorption losses in its coating and substrate, becomes transparent to light recirculating in the external cavity formed by the first and third mirrors, thus destabilizing the original cavity modes of the resonator formed by the first and second mirrors. In this case, even a small amount of absorption or scattering in the coating or substrate of the second mirror will result in the resonator formed by the first and third mirrors to have low net gain. This is because the inherent low gain of many types of gas lasers renders them particularly sensitive to intracavity loss. To address these issues, Knollenberg describes a method to modulate the external cavity along the laser axis, thereby creating a broad, Doppler induced, incoherent spectrum. This reduces the Q value of that cavity, because the nominal Q, as calculated from standard Fabry-Perot formulas, depends, for buildup of optical power, on having a resonance wavelength as opposed to a broad spectrum.
It is an object, therefore, of the present invention to provide a particle detector having increased sensitivity to detecting aerosol particles of sub-micron size.
It is a further object of the present invention to provide a laser source for a particle detector that is less sensitive to optical loss caused by detection of aerosol particulates.